The invention relates generally to photodetectors, such as for use in X-ray imaging applications. More particularly, the invention relates to a photodiode design for such detectors that reduces leakage of charge from the photodiode that can result from exposure of the photodetector to radiation over time.
Digital X-ray imaging systems have become increasingly important in a number of technical areas. Such imaging systems are presently used in medical applications, such as for projection X-ray, X-ray tomosynthesis, computer tomography systems, and so forth. Digital X-ray systems are also currently in use for part and parcel inspection, and other such non-medical uses. In general, digital X-ray imaging systems rely upon a stream of X-ray radiation from a source that impacts a detector array after traversing a subject or object of interest. The X-ray radiation is received by a scintillator in the detector, and charges in photodiodes are depleted by photons from the scintillator. The charge depletion can be measured, resulting in information for discrete picture elements (“pixels”) at the photodiode locations that can be analyzed for reconstruction of an image.
Flat panel amorphous silicon-based X-ray detectors of the type currently used in digital X-ray systems have excellent performance, but may degrade over time. One mechanism of such degradation involves the amorphous silicon photodiode which can become leaky as a function of X-ray dose. This leakage may be proportional to the area of the photodiode, the length of the periphery of the photodiode, or a combination of the two.
Where flat panel detectors are used for medical applications, dosages are typically prescribed based upon exposure limitations of both patients and medical staff. In other environments, however, such as for industrial part inspection, much stronger X-ray doses may be used. Such stronger doses tend to significantly reduce the life of conventional digital X-ray detectors by significantly increasing the leakage. Similar degradation occurs in medical applications, although over longer periods of time. As the leakage increases, a dark image offset value may be changed to accommodate the leakage, but this ultimately results in reduction of the dynamic range of the individual pixels. That is, the amount of charge between the dark image charge and the fully exposed charge becomes reduced, ultimately resulting in decommissioning of the detectors at the end of an abbreviated useful life.
There is a need, therefore, for an improved digital X-ray detector that can avoid problems with degradation of photodiodes due to exposure. There is a particular need for a photodiode design that can operate in conventional X-ray settings, while reducing or limiting leakage and the consequent degradation of the detector pixels and their performance owing to such leakage.